Conventionally, in an active-matrix display apparatus such as a liquid crystal display apparatus, a thin-film semiconductor device referred to as a thin film transistor (TFT) has been used. In the display apparatus, the TFT is used as a switching device for selecting a pixel or as a driving transistor for driving a pixel.
Recently, an organic electroluminescence (EL) display using electroluminescence (EL) of an organic material has been attracting attention as one of next-generation flat panel displays to replace the liquid crystal display. The organic EL display, unlike the liquid crystal display that is voltage-driven, is a current-driven device, so that it is urgently expected to develop a thin-film semiconductor device having an advantage in ON-OFF characteristics as the driving circuit for the active-matrix display apparatus.
Conventionally, as a thin-film semiconductor device for the driving circuit of the liquid crystal display, there is a thin-film semiconductor device in which an amorphous semiconductor layer, as a single layer, is used for a channel layer. This type of thin-film semiconductor device has a problem of low ON-current (drain current at the time of turning ON the gate) despite low OFF-current (leakage current at the time of turning OFF the gate), due to low charge mobility caused by a large bandgap of the channel layer.
To deal with this problem, for example, Non-Patent Reference 1 (Hatzopoulos et al., IEEE ELECTRON DEVICE LETTERS 28, 803 (2007)) discloses a thin-film semiconductor device in which the channel layer is formed in a two-layer structure consisting of a polycrystalline semiconductor layer and an amorphous semiconductor layer. By thus forming the channel layer into such a two-layer structure of the polycrystalline semiconductor layer and the amorphous semiconductor layer, it is considered that ideally, interaction between advantages of these two layers would increase the ON-current compared to the thin-film semiconductor device in which the channel layer is a single layer formed as an amorphous semiconductor layer, and such interaction would also decrease the OFF-current compared to the thin-film semiconductor device in which the channel layer is formed as a single polycrystalline semiconductor layer.
In addition, as the thin-film semiconductor device, there is a thin-film semiconductor device of a channel protection type (etching stopper type) that protects the channel layer from etching processing, but the thin-film semiconductor device of the channel protection type has a problem of a back channel being generated by a positive fixed charge in a channel protection film and accordingly generating a leakage current.
To deal with this problem, Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 2011-71440) discloses a thin-film transistor of the channel protection type in which: a gate electrode, a gate insulating film, an active semiconductor film having a two-layer structure of a first semiconductor film (polycrystalline semiconductor layer) formed of microcrystalline silicon, polysilicon, and the like, and a second semiconductor film formed of amorphous silicon (amorphous semiconductor layer), and a back channel protection insulating film are serially formed above a substrate. The Patent Reference 1 discloses an effect of suppressing the back channel effect due to the fixed charge in the channel protection film.